The present invention relates to a maximum-ratio synthetic transmission diversity device suitable for use in mobile communications, such as communications carried out by a personal handy-phone system (PHS).
An array antenna is one type of conventional transmission antenna and comprises phased-array antennas, adaptive-array antennas, or the like. Another type of conventional transmission antenna is an antenna selection diversity antenna. The phased-array antenna usually has a configuration such as that shown in FIG. 8.
In the drawing, reference numeral 1 designates a plurality of antenna elements; 2 designates a phase shifter; 3 designates an antenna multiplexer (or switch); 4 designates a receiver; 5 designates a transmitter; and 6 designates a control section. The phase shifter 2 has the function of controlling the phase of a transmission signal and is provided for a power feeding section of each antenna element 1. The control section 6 controls the phase shifters 2.
The phase of a signal to be transmitted to each antenna element 1 is adjusted by the control section 6 controlling each phase shifter 2 so as to synthesize the phases of the transmission signals in space, thus forming a wave beam 7 in a predetermined direction and improving the gain of the antenna.
In such a case, there exists a need to arrange the antenna elements 1 at intervals of xcex/2 (xcex is a wavelength of a wave to be used) or less. Taking the number of antenna elements 1 as N, the gain of the antenna in the predetermined direction can be improved by a factor of N.
As mentioned above, although the wave beam formed by the phased-array antenna can be adaptively controlled depending on a change in a wave environment, the wave beam is not widely utilized, because of its inherent problems, such as the length of an adaptive time or the accuracy of the phase shifter.
To eliminate these drawbacks in the conventional phased-array antenna, an adaptive array technique has already been developed. FIG. 9 shows an exemplary configuration of a conventional adaptive array antenna. In the drawing, reference numeral 10 designates a plurality of antenna elements; 11 designates an antenna multiplexer; 12 designates a transmitter; 13 designates a receiver; 14 designates a digital signal processing section; 15 designates a phase-and-power detection section; 16 designates a transmission signal generation circuit; and 17 designates a control section.
As shown in FIG. 9, the adaptive array antenna also comprises the plurality of antennas 10 arranged at intervals xe2x80x9cdxe2x80x9d equal to or less than xcex/2, as in the case of the phased-array antenna. A signal received by each antenna element 10 is demodulated by the receiver 13, and the control section 17 calculates the phase and power of the transmission signal on the basis of the phase and power of the signal detected from the demodulated signal by the phase-and-power detection section 15. Depending on the thus-calculated phase and power of the transmission signal, the transmitter 12 demodulates a transmission signal generated by the transmission signal generation circuit 16. The thus-demodulated signals are fed to the antenna elements 10, so that the transmission signal is synthesized in space. This adaptive array technique solves the inherent drawbacks of the phased-array antenna, such as the length of an adaptive time or the accuracy of phase control.
In order to reduce an inter-antenna correlation coefficient, the antenna selective diversity device comprises a plurality of antenna elements arranged at intervals of xcex/2 or more and adopts a method of selecting an antenna element to be used for transmission on the basis of the level of the power received by the plurality of antenna elements or the like. A relationship between the correlation coefficient and the interval among the antenna elements assumes a curve such as that plotted in FIG. 10. In a case where the antenna elements are arranged at intervals of xcex/2 or more, the correlation coefficient among the antenna elements can be reduced. However, in such a case, since the individual antenna elements are susceptible to a varying fading phenomenon, the influence of the fading phenomenon on the antenna elements can be diminished by selection of the antenna elements in a manner as shown in FIG. 11. In FIG. 11, the horizontal axis represents time and the vertical axis represents a receiving level of each antenna element.
The conventional transmission antennas mentioned previously suffer the following problems:
First, the phased-array antenna is intended to form a wave beam in a predetermined direction, and hence the antenna elements must be arranged at intervals of xcex/2 or less. Because of such a configuration, there is a high correlation coefficient among the antenna elements, and the antenna elements are subjected to the influence of a fading phenomenon, thus deteriorating the characteristics of the phased-array antenna.
The antenna selective diversity device employs a method of transmitting a signal by selection of one antenna element from a plurality of antenna elements on the basis of the power of signals received by the antenna elements. When compared with the gain of the array antenna obtained through synthesis of phases, there is a slight improvement in the receive power of a mobile terminal (i.e., an improvement in the gain of the antenna). Particularly, in a fading-free environment of superior visibility, there is no improvement in the receive power of the terminal.
The object of the present invention is to overcome the foregoing drawbacks of the conventional array antennas and those of the conventional antenna selection diversity devices mentioned previously, as well as to provide a maximum-ratio synthetic transmission diversity device which permits an improvement in an antenna gain of an array antenna and accomplishment of a space diversity effect stemming from a reduction in the correlation among antenna elements.
To accomplish the foregoing object, a maximum-ratio synthetic transmission diversity device, according to the present invention, comprises a plurality of antenna elements which are arranged intervals greater than xcex/2 (where xcex represents the wavelength of a wave to be used); a plurality of transmitters and receivers provided so as to correspond to the respective antenna elements; antenna multiplexing means for selectively connecting the antenna elements with one of the receivers and transmitters, respectively; and signal processing means which detects the phase of the signal received by each of the receivers and sends a transmission signal having a phase corresponding to the result of such detection to each of the transmitters, where the transmission signal is transmitted by way of each of the antenna elements.
A personal handy-phone system, according to the present invention, uses a maximum-ratio synthetic transmission diversity device as a base station, the diversity device comprising a plurality of antenna elements which are arranged intervals greater than xcex/2 (where xcex represents the wavelength of a wave to be used); a plurality of transmitters and receivers provided so as to correspond to the respective antenna elements; antenna multiplexing means for selectively connecting the antenna elements with one of the receivers and transmitters, respectively; and signal processing means which detects the phase of the signal received by each of the receivers and sends a transmission signal having a phase corresponding to the result of such detection to each of the transmitters, where the transmission signal is transmitted by way of each of the antenna elements.
In each of the foregoing inventions, in addition to the phase of the signal, the power of the received signal may be detected, and a transmission signal having a phase and power corresponding to the result of such detection may be sent to each of the transmitters, where the transmission signal is transmitted by way of each of the antenna elements.